Optical waveguides are used in industry and science in a variety of processes and purposes and have been proposed for use in supporting and illuminating photocatalysts. Mariangeli, R. E. and D. F. Ollis, AIChE J. 23 (4): 1000 (1977). In this configuration they could be used in remediating effluent waste streams (e.g., water or air) by photocatalytic oxidation or photooxidation. Titanium dioxide (TiO.sub.2) and other transition metal oxides are well-known as effective photocatalysts that can oxidize organic compounds to CO.sub.2 and H.sub.2 O in the presence of UV light (at wavelengths of about 380 nm or less for TiO.sub.2) and a suitable electron acceptor, such as O.sub.2. Metal oxide-mediated photocatalytic processes can occur at ambient temperatures. The coated waveguides can also remove inorganic ions from solution and can find utility in processes for converting ionic species into neutral species, such as metals.
Several shortcomings in metal oxide-based photocatalytic processes have been identified. For instance, (a) the ratio of illuminated catalyst surface area to reactor volume is often low, (b) the photocatalyst must be fixed in the reactor to separate from the reactant, and (c) the photocatalyst uses the activating ultraviolet radiation inefficiently. For example, UV light distribution throughout a typical TiO.sub.2 packed-bed reactor design is hindered by the high UV absorptivity of TiO.sub.2 and by losses due to reflection and scattering. Attempts to overcome these limitations have generally not succeeded.
For example, TiO.sub.2 -coated optical fibers have been used for photocatalytic oxidation of organic compounds in water. In such systems, UV light is propagated through an optical fiber substrate to photoactivate the TiO.sub.2 coating. The UV light is not completely absorbed in a single coated region of the fiber. TiO.sub.2 -coated optical fibers as developed thus far are not an adequate solution to the identified shortcomings in that it has only been possible to propagate UV light for about 10-15 cm. Peill, N. J., and Hoffmann, M. R., "Mathematical Model of a Photocatalytic Fiber-Optic Cable Reactor for Heterogeneous Photocatalysis," Environ. Sci. Technol., 32:398-404 (1998); Peill, N. J., and Hoffmann, M. R., "Development and Optimization of a TiO.sub.2 -Coated Fiber Optic Cable Reactor: Photocatalytic Degradation of 4-Chlorophenol," Environ. Sci. Technol., 29:2974-81 (1995).
Peill and Hoffmann determined that at each reflection at the fiber/TiO.sub.2 interface a portion of the UV light was refracted out of the fiber and absorbed by the TiO.sub.2 coating. Successive reflections quickly diminished the UV light intensity in the fiber. According to Peill and Hoffmann, the UV light propagated through the optical fibers in a frustrated total reflection (FTR) mode which is expected when light is incident from an optically rarer medium (i.e., silica) to an optically denser medium (i.e., TiO.sub.2). Peill and Hoffmann point out that "[t]he refractive index of TiO.sub.2 is higher than that of fused-silica glass . . . . For this reason, it is impossible that total refection occurs at the interface . . . the light flux is divided: one part of it is reflected and the other part leaves the fiber." Peill and Hoffmann, (1998), supra.
In another approach, U.S. Pat. Nos. 5,194,161 and 4,997,576 disclose processes for oxidizing organic compounds in an oil film floating on water. These patents describe coating photocatalytic metal dioxides, including TiO.sub.2, onto water-floatable waveguiding materials such as silica beads. UV light trapped in the coated bead is scattered onto the photocatalytic material where it is completely absorbed so as to create a photon flux for photocatalytic oxidation of the oil by oxygen. Although the patents envision using the coated materials to oxidize organic compounds, the patents describe preparing the coated materials so as to rapidly absorb as much trapped UV light as possible from the substrate. This approach is contrary to the articulated desire to improve the efficiency with which UV light is used in photocatalytic systems.
The art is still in need of a photocatalytic system that efficiently propagates UV light in a coated photocatalytic waveguide. Such a waveguide would allow for the controlled interaction of light energy with a large photocatalyst surface area, thus, enhancing the efficiency of heterogeneous photocatalytic processes, including but not limited to photooxidation or solar energy conversion. Such waveguides could also be the basis for novel optical chemical and biochemical sensors.